1. Field of the Invention
The invention relates to a process for producing compositions having improved gas barrier properties, compositions derived from the process, compositions containing polyester, compositions containing a polyester and a filler, and containers made from the compositions including clear, mono-layer beverage bottles. The invention further relates to polyester compositions, and films derived from the polyester compositions, having improved gas barrier characteristics.
2. Description of the Related Art
Polyester resins including resins such as poly(ethylene terephthalate) (PET), poly(butylene terephthalate) (PBT), poly(ethylene naphthalate) (PEN), poly(trimethylene terephthalate) (PTT), and poly(trimethylene naphthalate) (PTN), are conventionally used as resins in the manufacture of containers such as beverage bottles. Properties such as flexibility, good impact resistance, and transparency, together with good melt processability, permit polyester resins to be widely used for this application.
The starting feedstocks for polyester resins are petroleum derivatives such as ethylene, which is obtained from petroleum or natural gas, and para-xylene, which is typically obtained from petroleum.
Polyester resins are generally made by a combined esterification/polycondensation reaction between monomer units of a diol (e.g., ethylene glycol (EG)) and a dicarboxylic acid (e.g., terephthalic acid (TPA)). The terms carboxylic acid and/or dicarboxylic acid, as used herein, include ester derivatives of the carboxylic acid and dicarboxylic acids. Esters of carboxylic acids and dicarboxylic acids may contain one or more C1-C6 alkyl groups (e.g., methyl, ethyl, propyl, iso-propyl, butyl, iso-butyl, tert-butyl, pentyl, hexyl and mixtures thereof) in the ester unit, for example, dimethyl terephthalate (DMT).
In conventional esterification/polycondensation processes, PET may be formed, for example, by first producing a prepolymer of low molecular weight and low intrinsic viscosity (IV) (e.g., a mixture of oligomers), for example, by reacting a diol and a dicarboxylic acid in a melt phase reaction. The formation of the oligomers may be carried out by reacting a slurry of diol and dicarboxylic acid monomer units in an esterification reactor. EG may be lost to evaporation during the esterification reaction which may be carried out at high temperatures. Therefore the slurry of diol and dicarboxylic acid may contain an excess of EG, for example the diol and dicarboxylic acid may be present in a molar ratio of from about 1.2 to about 2.5 based on the total glycol to total di-acid. Further pre-polycondensation and polycondensation of the oligomers can be carried out to provide a resin mixture having an IV of from 0.50 to 0.62. Such resin mixtures are suitable in various applications such as fibers/filaments, fiber chips, or bottle-resin precursors. Amorphous clear base chips having an IV of from 0.50 to 0.60 may be subjected to solid-state polymerization (SSP) to increase the molecular weight (e.g., to an IV of from 0.74 to 0.76 for water bottle applications, 0.84-0.85 for CSD/beer bottles, etc.). The solid-state polymerization process unit can result in the resin undergoing crystallization which forms opaque pellets.
A continuous polyester (e.g., PET) melt-phase polycondensation process usually consists of three reaction steps: (i) esterification to form low molecular weight oligomers, (ii) pre-polymerization of the oligomers to form a pre-polymer, and (iii) post-polymerization to form a polymer with an intermediate molecular weight or intrinsic viscosity (e.g., a target intrinsic viscosity of from 0.57-0.62).
The three reaction steps (i), (ii), and (iii) above, can be carried out to achieve the target intrinsic viscosity in from 3 to 6 reactors using existing melt-phase process technology. In general, esterification is conducted in one or two vessels to form a mixture of low molecular weight oligomers with a low degree of polymerization (e.g., about up to 7 monomer unit pairs reacted). The oligomers are then pumped to one or two pre-polymerization vessels where higher temperatures and lower pressures aid in removing water and EG. The degree of polymerization then increases to a level of 15 to 20 repeating units. The temperatures are further increased and pressures are further reduced in the final one or two vessels to form a polymer ready to be cut into pellets for example, or to be spun directly into fibers or filaments.
Esterification and pre-polymerization vessels may be agitated. Polycondensation vessels (e.g., finishers, wiped-film reactors etc.) may have agitators designed to generate very thin films. Temperatures and hold-up times are optimized for each set of vessels to minimize the side and degradation reactions. Some by-products that may be generated by the PET melt phase reaction include diethylene glycol (DEG), acetaldehyde, water, cyclic oligomers, carboxyl end groups, vinyl end groups, and anhydride end groups.
Both time and temperature are two variables that are preferably controlled during an esterification/polycondensation reaction. With higher reaction temperatures, the total reaction time is significantly reduced and fewer reactors are needed.
Alternatively to such a continuous production method, polyesters may be prepared using a batch method. In a batch method the diol and dicarboxylic acid units are mixed together in a single reactor. In some cases more than one reactor (e.g., reaction vessel) may be used if necessary. The diol/dicarboxylic acid mixture is heated to cause the monomer units to undergo a condensation reaction. The by-products of the condensation reaction may include water or an alcohol. By conducting the reaction under reduced pressure or by subjecting the reaction mixture to reduced pressure during the final stages of the reaction, volatile by-products of the reaction can be removed thus driving the reaction to completion.
Certain physical and chemical properties of polymeric materials are negatively affected by long exposure to elevated temperature, especially if the exposure is in an oxygen-containing atmosphere or at temperatures above, for example, 250° C. Conventional methods for preparing polyester resins such as PET may suffer from disadvantages associated with the need to carry out an SSP which subjects the resin to a long heat history and/or may require high capital expenditure.
The production of a polyester resin such as PET may be carried out directly from a melt phase of the monomer units without any final solid-state polymerization. For example, a batch process may be carried out at a sufficient temperature, for a sufficient time and at a sufficient pressure to drive the polycondensation reaction to completion thus avoiding the need for any subsequent finishing (e.g., final reaction).
Solid-state polycondensation (SSP) is an important step in some conventional processes used to manufacture high molecular weight PET resins for bottle, food-tray, and tire-cord applications. The clear amorphous pellets (0.57-0.62 IV) produced by the conventional polycondensation reaction processes discussed above may be further polymerized in the solid state at a temperature substantially higher than the polymer glass transition temperature but below the crystalline melting point. The solid state polymerization is carried out in a stream of an inert gas (usually nitrogen under continuous operation) or under a vacuum (usually in a batch rotary vacuum dryer). At an appropriate SSP temperature, the functional end groups of the PET chains are sufficiently mobile and react with one another to further increase the molecular weight.
A conventional process for producing polyester resins for container applications including melt-phase polycondensation and solid state polymerization is shown schematically in FIG. 1 wherein the monomer components of a polyester resin such as PET are mixed in a melt-phase esterification/polycondensation reactor. The reaction is carried out to provide a molten resin having an intrinsic viscosity (IV) of from 0.5 to 0.6. The molten product obtained by the melt-phase esterification/polycondensation is then subjected to a polymer filtration. Optionally a co-barrier resin may be added to the filtered, molten polymer by extruding the co-barrier resin and adding the extrudate to the filtered, molten resin obtained from the melt-phase esterification/polycondensation. The mixed streams, or the polyester stream obtained from polymer filtration may then be pumped into a mixer. A static mixer may be used to ensure that the polyester resin and any co-barrier resin are sufficiently mixed.
The melt-phase esterification/polycondensation is typically carried out in a plurality of reactors. Therefore, the monomers may be added to a first esterification reactor to form a low IV material. As the oligomers pass through the remaining reactors, the IV is subsequently raised as the polycondensation reaction proceeds sequentially through a series of reactors. The material in molten form that is pumped from the static mixer is subjected to solidification and pelletizing. The molten material may be solidified by passage of strands or filaments of the material formed by pumping the material through, for example, a die with a series of orifices. As the molten polyester resin is passed through an orifice, a continuous strand is formed. By passing the strands through water, the strands are immediately cooled to form a solid. Subsequent cutting of the strands provides pellets or chips which, in a conventional process, are then transferred to a solid-state polymerization stage (SSP).
In conventional processes for preparing PET, and even in processes which avoid the use of a solid-state polymerization, after polymerization is complete, the molten polymerized resin is pumped through a die to form multiple strands. The molten resin exiting from the die is quickly quenched in water to harden the PET or polyester resin. As a result of the quick cooling (e.g., water quench) the molten polyester does not have time to crystallize and is solidified in an amorphous state. Solidified PET strands, or pellets derived from cut strands, are clear, transparent and in an amorphous state.
The SSP may include several individual reactors and/or processing stations. For example, the SSP may include a pre-crystallization step wherein the chips and/or pellets are transformed from an amorphous phase into a crystalline phase. The use of a crystalline phase polyester resin is important in later steps of the SSP because the use of amorphous polyester chips may result in clumping of the pellets since an amorphous state polyester resin may not be sufficiently resistant to adherence between pellets and/or chips. The SSP process further includes a crystallizer (e.g., crystallization step), a pre-heater, and an SSP reactor.
Some manufacturing processes do not include an SSP. Processing a polyester resin directly from a melt phase condensation to obtain pre-forms for blow molding applications is described in U.S. Pat. No. 5,968,429 (incorporated herein by reference in its entirety). The polymerization is carried out without an intermediate solidification of the melt phase and permits the continuous production of molded polyester articles (e.g., pre-forms), from a continuous melt phase reaction of the starting monomers.
After pre-crystallization, the chips and/or pellets may be subjected to a final crystallization. A final crystallization may include, for example, proper heating of the chips (pellets, pastilles, granules, round particles, etc.) at appropriate temperatures. Once the polyester resin is in a crystallized state, the pellets and/or chips are preheated and ready for transfer to the top of a counter-flow SSP reactor (parallel to the pre-heater) via a pneumatic system (e.g., Buhler technology). If a tilted crystallizer is stacked above the SSP reactor, the hot/crystallized chips then enter the SSP reactor by its rotating screw of the crystallizer (e.g., Sinco technology). The SSP reactor can be considered as a moving bed of chips that move under the influence of gravity. The chips have a slow down-flow velocity of about 30 mm/minute and the nitrogen has a high up-flow velocity of about 18 m/minute. A typical mass-flow ratio of nitrogen to PET is in the range of 0.4-0.6. In a gravity-flow reactor, the pellets and/or chips are subjected to elevated temperatures for periods of up to 15 hours. The heating and nitrogen sweeping through the gravity-flow reactor will drive the polycondensation reaction and result in longer chain lengths and, concurrently, a higher IV of the resins.
After passing through the gravity-flow reactor, pellets and/or chips having an IV of about 0.84 may be formed. The pellets and/or chips have an opaque characteristic due to their crystallinity. The crystalline material is transferred to a product silo for storage and/or packaging. The finished product in a crystalline state and having a IV of about 0.84, can be further mixed with other co-barrier resins (powders, granules, pellets, pastilles, etc.) by molders or processors who purchase the polyester resins for manufacturing, for example, bottles and/or containers.
Thus, in a conventional process, a melt-phase polycondensation process may be used to make clear amorphous pellets (typically, 0.5-0.6 IV) as precursors to bottle resins. The amorphous pellets are first pre-crystallized, crystallized, and/or preheated, then subjected to SSP in a gravity flow reactor (e.g., a reactor that is not agitated). After crystallization, the resin pellets become opaque and do not stick together if the temperature of SSP is at least 110° C. below the onset of the melting temperature of the resin pellets. In a direct high IV process, only the melt process (no SSP) is used to make a variety of bottle resins (e.g., 0.75 IV for water bottles, 0.85 IV for CSD/beer bottles) as desired. A finisher (e.g., a wiped-film evaporator) may be used to effectively and rapidly remove the reaction by-products such as EG (major), water, acetaldehyde, and so on in direct high IV processes. Immediate removal of EG/water under high temperatures drives the polycondensation reaction equilibrium toward the polymer side.
PET or other polyester resins are known to have hygroscopic behavior (e.g., absorb water from the atmosphere), so pellets obtained by cutting water-quenched strands contain significant quantities of water. Conventionally, the pellets may be dried by passing dry air over the pellets or by heating. Heating for an extended period at an elevated temperature may lead to problems because the amorphous polyester (e.g., PET) pellets may have a tendency to stick to one another.
In preform molding processes, the pellets and/or chips are typically dried before molding. After proper drying, the pellets and/or chips may have a water content of around 50 ppm. The chips and/or pellets are then processed, for example, in the form of pre-forms, by injection molding. Because water is present during the injection molding process which is carried out at elevated temperatures (e.g., temperatures above 200° C.), the IV of the resin may be reduced. The starting chips may be about 0.84 IV. The IV in subsequent injection-molded preforms formed from the starting resin may be about 0.80 IV. Thus, an approximate 5% reduction in IV of about 0.04 units may take place in going from the chips and/or pellets to the pre-form prepared by injection molding when the chips and/or pellets have been properly dried and contain at most about 50 ppm water. Polyester material containing a greater amount of water can undergo thermal and hydrolytic degradation. Excess water in the resin can lead to a substantial reduction in IV of 30% or more.
Conventionally, the pre-form is transformed to a bottle or a container through a blowing operation. The blowing is carried out at a temperature above the glass transition temperature of, for example, 90-110° C. which is substantially lower than the injection molding temperatures to which the pellets and/or chips are exposed during injection molding to form the pre-form. Pre-heating a pre-form is often provided in the form of an infrared heater. Thus the IV of the resin may not change substantially, and preferably does not change at all, during the blow molding process.
An important property of any polymer resin used in food container or beverage container applications is the resin's ability to resist the ingress and egress of gases through the container's walls. Containers for carbonated beverages may be especially susceptible to the egress of gases such as carbon dioxide which is normally present in carbonated soft drinks. Usually, a carbonated soft drink will contain about 4 volumes of dissolved carbon dioxide gas per volume of the liquid carbonated soft drink. Other beverages such as beer typically have approximately 2.8 volumes of total dissolved carbon dioxide.
If the resin material used to form a beverage container permits carbon dioxide to escape, the product delivered to the consumer may be of unacceptable quality (e.g., “flat”) if stored too long. In food container applications it is important to resist the ingress of oxygen. Oxygen in contact with a food substance may lead to oxidation and accelerated staleness of the food product.
U.S. Published Application No. 2000/0029712 describes a method that includes the formation of polyester resins directly from a melt phase without any intermediate solid state polymerization. The polymer compositions derived from the process may not exhibit the gas barrier resistance necessary for most modern food and/or beverage container applications. Therefore, a secondary resin layer such as a layer of nylon or an ethylene vinyl alcohol (EVOH) polymer must be used in order to prepare a two layer beverage container of acceptable gas permeation properties.
Some multi-layer food and/or beverage containers may exhibit the required resistance to gas permeation necessary to make the resins acceptable for these applications. There is substantial additional cost and complexity associated with preparing a dual-layer or tri-layer container in comparison to a single-layer container. Such costs are related to the need for additional and more sophisticated processing equipment and technical issues such as delamination between the layers making up the inner and outer surfaces of the container.
Thus, there is a need for a process that combines the advantages of a continuous production process for forming a pre-form for blow molding directly from a melt phase resin obtained by melt condensation (without the need for an intermediate SSP step), with the ability to obtain a resin from that process that may be used to form a single-layer container that exhibits improved resistance to gas permeation.